Illegal expression in contains

src/operators.js

@@ -5,7 +5,7 @@ var operators = {
     '<': (l, r) => l < r,
     '>=': (l, r) => l >= r,
     '<=': (l, r) => l <= r,
-    'contains': (l, r) => l.indexOf(r) > -1,
+    'contains': (l, r) => typeof l === 'string' ? l.indexOf(r) > -1 : false,
     'and': (l, r) => l && r,
     'or': (l, r) => l || r
 };

test/syntax.js

@@ -13,7 +13,8 @@ describe('expression', function() {
         scope = Scope.factory({
             one: 1,
             two: 2,
-            x: 'XXX'
+            x: 'XXX',
+            y: undefined
         });
     });
 
@@ -25,6 +26,7 @@ describe('expression', function() {
     it('should throw on illegal expression', function() {
         expect(function() {
             evalExp('1 contains "x"', scope);
+            evalExp('y contains "x"', scope);
         }).to.throw();
     });